This invention relates to an improvement to an X-ray tube apparatus, and more specifically to oscillation or vibration absorbing means for the X-ray tube apparatus.
X-ray tube apparatuses generally include a housing in which insulating oil is sealed, a rotary anode X-ray tube (hereinafter referred to as the "X-ray tube") placed in the housing and supported by a support and a stator fixed to the housing and forming a motor in cooperation with a rotor placed in the X-ray tube. The X-ray tube consists of a glass bulb maintaining a vacuum inside, with a sleeve-like journal box fixed at one of the ends of the bulb so as to extend inwardly in the axial direction. The journal box supports, via ball bearings, the rotor to which an anode target is fixed. The rotor is positioned so as to oppose the stator via the wall of the glass bulb. A cathode is fixed at the other end of the glass bulb. A part of the cathode opposes the anode target and projects the electron beam to the anode target so that the X-rays are emitted from the surface of the anode target.
When the electron beam is radiated to the anode target, it attains an average temperature of about 1,200.degree. C. Since the inside of the glass bulb is in high vacuum, most of the heat is radiated and transferred to the outside. However, a part of the heat of the anode target is transmitted to the shaft, to the ball bearings, and then to the journal box, and the temperature of the journal box reaches about 500.degree. C. In view of thermal expansion, therefore, ball bearings having a bearing gap ranging from 30 .mu.m to 60 .mu.m (compared to 5 to 10 .mu.m in ordinary motors in general) are generally employed. In the room temperature environment at the initial stage of rotation, the gap between the ball bearings is so great that the anode target causes unstable rotation oscillation as well as large rotation noise. Especially in a critical speed range in which rotating oscillation rapidly increases, an abnormal load acts upon the ball bearings and the latter are frequently damaged prematurely.
As methods of reducing the dynamic load acting upon the ball bearings, Japanese Patent Publication No. 12162/1970, Japanese Patent Laid-Open No. 57786/1974 and Japanese Patent Laid-Open No. 44691/1974 propose a construction which sets the critical speed to a lower level by reducing the support rigidity of the rotation system. These proposals are effective for reducing the critical speed of the rotation system and mitigating the dynamic load due to the mass unbalance that acts upon the ball bearings. When the full speed range is taken into account, however, they are not yet sufficient to prevent damage to the ball bearings. This can be confirmed from the fact that when the rotating oscillation characteristics of the X-ray tube are actually measured, rotating oscillation rapidly increases in a high speed range after passing through the critical speed range and exhibits unstable oscillation characteristics even in a flexible support structure.
According to an oscillation-proofing design for high speed rotary machines in general, an oscillation damping element or elements are disposed in the proximity of bearings so as to absorb abnormal or unstable oscillation. However, since the X-ray tube is placed in the specific environment of high vacuum and high temperature, ordinary damping means using oil film dampers or oscillation-proof rubbers can not be used in the X-ray tube. Though a solid friction damper can be used, the friction surface is likely to catch due to the high temperature and high vacuum condition, and the damper soon loses its function.
Oscillation-proofing of the anode target is necessary for extending the life of the ball bearings and for reducing the noise of the rotation sound. Especially when oscillation of the anode target becomes great, focusing of the X-rays is likely to deviate and satisfactory picture quality can not be obtained. If the apparatus is of a micro-small focusing type, excessive oscillation results in a critical problem in X-ray photography.